Due to its robustness to multipath fading and high spectral utilization efficiency, Orthogonal Frequency Division Multiplexing (OFDM) is a very attractive technique for high rate wireless data communications [J. A. C. Bingham, “Multicarrier modulation for data transmission: An idea whose time has come”, IEEE Commun. Mag., vol. 28, pp. 5-14, May 1990], [Y. Wu and W. Y. Zou, “COFDM: an overview”, IEEE Trans. On Broadcasting, vol. 41, pp. 1-8, March 1995]. It has been adopted for many applications such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB) and wireless Local Area Networks (LANs). However, its main disadvantage is the exhibiting of a prohibitively large peak-to-average power ratio (PAPR), which results in significant in-band distortion and spectral spreading when passed through a nonlinear device such as a transmitter power amplifier [A. R .S. Baiha, M. Singh, A. J. Goldsmith, and B. R. Saltzberg, “A new approach for evaluating clipping distortion in multicarrier systems,” IEEE J. Select Areas Commun, vol. 20, no. 5 pp., pp. 1037-1046, June 2002]. Without the use of any PAPR reduction technique, the efficiency of power consumption at the transmitter becomes very poor.
Different schemes have been proposed to mitigate PAPR problem. These are mainly classified into two categories: signal distortion and signal scrambling. With signal distortion techniques, OFDM signals are deliberately distorted prior to amplification. Among these, envelope clipping is the simplest. However, clipping introduces both in-band and out-band radiation [H. Ochiai and H. Imai, “Performance analysis of deliberately clipped OFDM signals,” IEEE Trans. On Commun., vol. 50, no. 1, pp 89-101, January 2002]. Although applying powerful coding schemes (e.g., near optimal turbo codes) can alleviate this problem, a performance loss is inevitable. With signal scrambling techniques, OFDM signals are modified distortionlessly to present better PAPR property. However, to recover the original OFDM signals correctly, side information should be provided, which might reduce data efficiency. Among many signal scrambling methods, the partial transmit sequence (PTS) scheme has been extensively studied due to its high performance and relatively low implementation cost [S. H. Muller and J. B. Huber, “OFDM with reduced peak-to-average power ratio by optimum combination of partial transmit sequence”, Electronic Lett., vol 33, no. 5, pp. 368-369, Febuary 1997]. In this scheme, OFDM sequences are partitioned into subblocks, and each subblock is multiplied by a phase weighting factor to produce alternative sequences with low PAPR. To reduce the complexity of exhaustive searches for best phase weighting factors, several suboptimal schemes were proposed [L. J. Cimini Jr. and N. R. Sollenberger, “Peak-to-average power ratio by optimum combination of partial transmit sequences”, IEEE Commun. Lett., vol 4, no. 3, pp 86-88, March 2000], [C. Tellambura, “Improved phase factors computation for the PAR reduction of an OFDM signal using PTS”, IEEE Commun. Lett., vol. 5, no. 4, pp. 135-137, April 2001]. It was shown in Cimini that an additional PAPR reduction by increasing the number of phase weighting factors (greater than 4) is negligible. As a matter of fact, a larger number of phase weighting factors increases the hardware complexity significantly and makes the whole system vulnerable to the effect of phase noise.
In this invention, a signal scrambling method based on combined symbol rotation and inversion (CSRI) is disclosed for PAPR reduction of OFDM signals. It was found in [M. Tan, J. Cheng, and Y. Bar-Ness, “OFDM peak power reduction by a novel coding scheme with threshold control,” Proc. IEEE VTC, vol. 2, pp. 669-672, October 2001] that by performing symbol rotation, high degrees of freedom are available to offset the possibility of encountering poor sequences with large PAPR. There, BPSK modulated OFDM blocks were encoded by odd parity checking codes of rate ¾, which could prevent producing a string of same bits. For a coded OFDM block of length N, performing bit rotation in each subblock, termed redundant bit position rotation (RBPR) where instead of putting redundant bit at the end of each subblock, by performing bit rotation, redundant bit is put at different positions in each subblock, 4N/4 different scrambled sequences can be produced and the one with the smallest PAPR is selected for transmission. The effectiveness of this scheme was exhibited by simulations. However, its complexity, which is exponentially increased with the subcarrier number, makes it unsuitable for practical application. By investigating the characteristic of PAPR distribution, a threshold control method was added in M. Tan, Cheng and Y. Bar-Ness, Op. Cit., which significantly reduced the complexity for finding the best sequence. Nevertheless, with a large number of subcarriers and small value of PAPR threshold, number of comparisons needed is still quite large. Therefore, suboptimal schemes are needed to further reduce complexity.